That depends of course how the transient/surge has been coupled into the
mains.
If that is due to a ligtning event at a short range,  the impedance may
be much less. 
Your analysis is right when the even has been generated far away.

Regards,

Ing.  Gert Gremmen, BSc



. 



-----Oorspronkelijk bericht-----
Van: John Woodgate [mailto:j...@jmwa.demon.co.uk] 
Verzonden: Saturday, September 07, 2013 11:35 AM
Aan: EMC-PSTC@LISTSERV.IEEE.ORG
Onderwerp: Re: [PSES] Spark Gap PCB Layout on AC Mains

In message <FCA549BE3ECF9D4CB8CB8576837EA48920A1A4@ZEUS.cetest.local>,
dated Sat, 7 Sep 2013, "ce-test, qualified testing bv - Gert Gremmen" 
<g.grem...@cetest.nl> writes:

>Anyway, the primary circuit always need to be build for high surge 
>currents,
>
>and one should always insert induction to limit surge currents.
>
>As the impedance of the spark gap is low, only a small amount of 
>induction
>
>will reduce the surge current substantially

Quite true, but the mains supply itself (unless it's from a very nearby
private MV/LV transformer, has inductance of the order of a millihenry
(0.3 ohms at 50 Hz), so there is not much point in adding only a few
microhenrys.

IEC TR60725 gives 'reference impedances' for various types of mains
supply. While these tend to be higher than the actual impedance at the
'point of common coupling', they are a guide to the impedance at a
typical wall-socket. This impedance limits the prospective short-circuit
current.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk If
dictionaries were correct, we would only need one, because they would
all give the same information.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

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